Polyethylene is divided into high-density (HDPE, density 0.941 g/cm3 or greater), medium-density (MDPE, density from 0.926 to 0.940 g/cm3), low-density (LDPE, density from 0.910 to 0.925 g/cm3), and linear low-density polyethylene (LLDPE, density from 0.910 to 0.925 g/cm3). See ASTM D4976-98: Standard Specification for Polyethylene Plastic Molding and Extrusion Materials. Polyethylene can also be divided by molecular weight. For instance, ultra-high molecular weight polyethylene denotes those which have a weight average molecular weight (Mw) greater than 3,000,000. See U.S. Pat. No. 6,265,504. High molecular weight polyethylene usually includes those which have an Mw from 130,000 to 1,000,000.
One of the main uses of polyethylene (HDPE, LLDPE, and LDPE) is in film applications, such as grocery sacks, institutional and consumer can liners, merchandise bags, shipping sacks, food packaging films, multi-wall bag liners, produce bags, deli wraps, stretch wraps, and shrink wraps. The key physical properties of polyethylene film include tear strength, impact strength, tensile properties, stiffness and transparency. Tensile properties are an important indicator of the material's behavior under loading in tension. Tensile properties include tensile strength at yield, tensile strength at break (ultimate tensile strength), tensile modulus (Young's modulus), and elongation at yield and break. High tensile strength at yield is required for heavy duty bags such as those for trash, top soil, and fertilizer. High tensile strength at yield means high resistance to deformation or elongation under loading.
Machine direction orientation (MDO) is known to the polyolefin industry. When a polymer is strained under uniaxial stress, the orientation becomes aligned in the direction of pull. For instance, U.S. Pat. No. 6,391,411 teaches the MDO of very high molecular weight (both Mn and Mw greater than 1,000,000) HDPE films. However, MDO of very high molecular weight HDPE films are limited because these films are difficult to stretch to a high draw-down ratio.
Polyethylene films, although having many desirable properties for the use in heavy duty bags, have low tensile strength at yield. Imagine that a person carries a 50-pound trash bag, while the bag elongates to five-foot long. She or he will feel unsafe and inconvenient. It would be desirable to prepare a polyethylene film which, along with other properties, has high tensile strength at yield so that the film can be used for heavy duty bags.